Wave-signal translating system



.IuIy 6, 4948. a. D. LouGHLlN 2,444,741

WAVESIGNAL TRANSLATING SYSTEM Filed Dec. 51, 1943 5 Sheets-Sheet 2 l I I I I I I FIG. 4 I I I I I I I I I l I I I I I e4 I I PHASE i INVERTER I L I 4 0. V I---n -'EIT T- f--"I 45 45 4,6 4,6 47 47 I I r I ll l ll l) I Il, l l 4|' $548 ige 4Q; `sa 43 5g; 5Q/,44;:I I f I I I 4I l l(h i u I s 55 I FISE f7' H0 I l I I T *l QINTEGRATING PHASE I I 4 G|RGU|T INVERTER o I f l; l I 1 .I C I I II II j i I l|9 12o E J -1 mi? g I Il I LV I I|1I I*.I|s -B'I rw I. L

INVENTOR BERNARD D. L OUGHLIN July 6, 4948. B. D. I OUGHLIN WAVE-SIGNAL TRANSLATING SYSTEM 3 Sheets-Sheet i5 Filed Dem. 3l, 1943 INVENTOR ERNAR D D. L

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Patented July 6, 1948 UNITED STA Bernard D. Loughlin, Bayside, N. Y., assigner, by mesne assignments, to Hazeltine Research, Inc., Chicago, lll., a corporation of Illinois Application December 31, 1943, Serial No. 516,394

7 Claims. (Cl. 177-353) 'This invention relates, in general, to wavesignal translating systems and is particularly dlrected to coding or secrecy aspects of transponder systems. As used throughout' this specification and in the appended claims,V the term transponder system" is intended to describe a wavesignal translating system which includes a. receiving system and a transmitting system so arranged that a predetermined answer or reply signal is transmitted in response to a received interrogating signal, the reply and interrogating signals preferably comprising radiant-energy wave signals.

Transponder systems of the type under conslderation are frequently utilized as-beacons for supplying directiOn-nding infomation to air trailic. In the usual installation the system is designed to translate pulse-modulated wave signals in a pulse-by-pulse fashion. That is, the receiving system is designed to receive a radiated interrogating signal having pulse-type characteristic variations, such as pulse-type amplitude variations, and the transmitting system is designed to transmit a reply signal having similar characteristic variations, their operation being such that each modulation pulse of the received interrogating signal causes the transmission of a single pulse of the reply signal. While transponder systems of the prior art have proved to be generally satisfactory, it may be desirable in particular installations to render the system secret and thus preclude unauthorized listeners from deriving useful information. The required secrecy may be obtained through the use of a transponder system having a receiver portion which responds only to received interrogating signals which are coded in accordance with a prescribed coding schedule. The prior art systems mentioned, arranged for a pulse-by-pulse response to received signals, do not lend themselves particularly well to such installations.

It is an object of the invention, therefore, to provide an improved Wave-signal transponder ysystem which substantially avoids the abovementioned limitation of prior art arrangements.

It is a further object of the invention to provide an improved transponder system for transmitting a reply signal, which may or may not be coded, in response to an interrogating signal having a plurality of pulse-type characteristic variations of predetermined pulse durations occurring in a predetermined time sequence.

It is a further object of the invention to provide a wave-signal transponder system which includes improved coding arrangements in its receiving system.

It is a still further object of the invention to provide a wave-signal transponder system having an improved signal-to-noise ratio.

In accordance with the invention, a selective wave-signal translating system for translating a signal having a plurality of puise-type amplitude variations of predetermined pulse duratlons occurring in a'predetermined time sequence comprises an input circuit for receiving the signal. Means are coupled to the input circuit for deriving from the received signal a first corresponding plurality of signal impulses occurring in time coincidence and individually representing the leading edge of a different one oi. said pulse-type amplitude variations of the received signal. Means are also provided for deriving from the received signal a second corresponding plurality of signal impulses occurring in time coincidence with one another and with the .first-mentioned. plurality of signal impulses and individually representing the trailing edge of a different one ci the pulse-type variations of the received signal. Finally, the system has means responsive jointly to the coincident rst and second plurality of signal impulses for deriving an output signal, Whereby the translating system is responsive only to signals having pulse-type amplitude variations of the aforesaid predetermined durations and occurring in the aforesaid predetermined time sequence.

For a better understanding cf the present lnvention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

Fig. 1 of the drawings is a schematic circuit diagram of a wave-signal transponder system having a selective receiver system; Figs. 2 and 3 comprise graphs utilized in explaining the operation of the arrangement of Fig. l; Fig. 4 represents a portion of the Fig. 1 arrangement modied to include the present invention; Fig. 5 cornprises a set of graphs utilized in explaining the operation of the Fig. 4 arrangement; Fig. 6 represents in modified form a further portion of thc Fig. 1 arrangement; and Fig. 'l comprises a set of graphs used in explaining the operation of the arrangement of Fig. 6.

Referring now more particularly to the drawings, there is represented in Fig. 1 a wave-signal transponder system having coding devices contributing desired secrecy features. For convenamic/n lence of explanation, the system may he considered as a radio beacon adapted to transmit direction-nding infomation to interrogating aircraft. As represented, the radio beacon includes a selective receiving system in accordance with the presentinvention having an antenna system I0. I-I coupled to a carrier-frequency signal -translator or receiver I2. Where the receiver is of the superhe'terodyne type, the conventional frequency v spectively, to limit the frequency components and amplitude of the signal applied to unit 2li in order that this signal may have a predetermined wave form. that is, a predetermined maximum amplitude and predetermined slopes at the leading and trailing edges of its pulse-type amplitude variations. The beacon further includes a'transmitting system which forms the subject matter of a divisional application Serial No. 557,953, led Ocf tober 10, 1944, now Patent 2,415,359, dated February 4, 1,947, in the name of Bernard D. Loughlin and assigned to the same assignee as the present invention. This transmitting system is coupled to the output circuit of multivibrator I5 and consists of a delay-network unit tu, described more particularly hereinafter, an amplifier I6, a carrier-frequency signal generator and modulator or ,transmitter I1, and a transmitting antenna It, It. Transmitter -Il' is normally in a. quiescent condition but isy energized in response to control signals translated through amplifier IE. Elemen'ts I0--I9, inclusive, may be of any conventional construction and operation. By way of illustration, amplitude limiter I3 may be similar to the limiter arrangement designated It of United States Letters Patent 2,271,203, granted January 27, 1942, in the name of Jasper J Okrent and assigned to the same assignee as the present invention. Clipping amplifier Id may constitute an amplier biased to pass only signal components y having an amplitude value exceeding a critical level, as illustrated at page 372, Fig. 217B of Principles of Television Engineering, by' Donald G. Fink, published in 1940 by McGraw-Hill Book Company, Inc. Multivibrator it may be a trigger circuit of the type shown at page 176, Figs. d-9 oi "Ultra-High-Frequency Techniques, by J. G. Brainerd et al., published in 1942 by D. Van Nostrand Company. Inc.

Neglecting for a moment units 2t and iii and considering element It to be a conventional ampliiier, it will be readily apparent to those skilled in the art that the arrangement of Fig. l cornprises a conventional radio beacon, that is, a signaling system having a receiver and transmitter interconnected so that the output signal of the receiver controls the transmitter to transmit a. related signal. Since the operation of such apparatus is generally well understood, a detailed description and explanation thereof is unnecessary.

Briey, however, anaircraft desiring to obtain infomation from the beacon sends out an 'interrogating signal having pulse-type amplitude variations of equal pulse durations and equal pulse spacings. This signal is intercepted by antenna system It, II and translated by receiver i2, the

modulation components of the signal appearing in the output circuit thereof as a unidirectional iii) Eli?

pulse of the interrogating signal.

signal having amplitude variations corresponding to those of the received signal. This signal is applied by way of elements I3 and I4 to keyed multivibrator it, which generates a single control pulse of rectangular wave form and of any desired pulse duration as determined by the circuit parameters of the multivibrator, one such control pulse being generated thereby in response to each modulation The control pulses from multivibrator I-5 are translated through ampliiier It, thereby to control transmitter il to transmit in response to each such control .pulse a single pulse of an answer or reply signal, the transmitted pulses having substantially the same pulse duration. spacings and wave form as those translated through amplifier I6. rlhus, the described transponder system, when interrogated by an inquiring aircraft, transmits a reply signal having pulse-type amplitude variations which individually correspond to modulation pulses of the interrogating signal. Expressed diierently, the beacon functions in a pulse-bypulse fashion to transmit an answer in response to an interrogating signal.

Referring now more particularly to the delaynetwork units 20 rand l!) of the Fig. 1 arrangement, these units are associated, respectively. with the receiver and transmitter portions of the transponder system. being provided to afford the desired secrecy characteristics. Unit 20 is a transversal time-delay filter network having input terminals 2i, 2l and .output terminals 22, 22. 'I'he delay network comprises a transmission li-neconsisting` oi' a pair of spaced parallel conductors 2l, 22, coupled at one end to input terminals 2i, 2i, and yterminated at the opposite end in the surge or characteristic impedance of the line designated 2d. The electrical characteristics of transmission line 23, are preferably such as to effect a uniform predetermined time delay per unit of line length to a signal translated thereby. The physical length of the line is determined by the degree of secrecy desired, as will presently be explained. A plurality of adjustable taps are associated with the line so as to be freely movable therealong, three such taps 25, lili and 2l being shown in the drawings. Each of taps 25, 26 and 21 is included in circuit with an individual one of isolating 'resistors Z9 and fifi and a common impedance 3| is coupled between output terminals 22, 22, of unit isolating resistor has a relatively high impedance as compared with'that ci common impedance 3 i.

Unit fill likewise comprises a transversal timedelay .lter network having input terminals d I 6I, output terminals d2, d2, a. parallel-conductor transmission line t3, coupled at one end to terminals di, il and terminated at the opposite end in its surge or characteristic impedance M. Transmission line dit, t3, may be of the same general construction and Adesign as line 23, 23, of unit 2d. Unit de also includes a plurality of adjustable taps stand lll adjustably connected to transmission line '53, d3, isolating resistors 4B, l

namely, tap 25 ci unit il! and tap 45 of unit 40. I

nal is translated directly from wave-signal translator I2 through unit I3, tap 25 of unit 2t and amplifier It to multivibrator I5. Also, the control pulses'generated in multivibrator IE are ltranslated directly therefrom through tap d5 of unit 4U to ampliiier I6 and transmitter I l. Accordingly, for these conditions the operation of the transponder system is substantially as described above.

Now it will be assumed that all 'of the taps of units 20 and 40 are eiiective and that such taps are positioned along their associated transmission lines in the manner indicated in the drawings. It' will be further assumed that clipping amplifier Id is adjusted to have a clipping level indicated by broken-line curve c of Fig. 2 and that the interrogating signal as applied to input terminals 2i, 2i, of unit 20 has the wave form represented by curveezs, of Fig. 2. From this last-mentioned curve, it 'will appear that the interrogating signal as applied to terminals 2|, 2i, has a maximum amplitude limited to a .predetermined value e by unit I3 and a group of ling of but a single group of such amplitude variations.

The response of the transponder system to such a received signal is illustrated by the curves of Figs. 2 and 3 in which the subscripts indicate the elements where the represented signal voltages are derived. Signal voltages are derived at each 'of taps 25, 26 and 21. If the signal attenuation along line 23, 23 be neglected, the derived signal voltages individually have a predetermined maximum amplitude equal to the abovementioned value e and pulse-type amplitude Y variations corresponding to the modulation of the received interrogating signal. Due to the time-delay characteristics of transmission line 23, 423, andthe relative spacings ofthe adjustable taps. these signal voltages have a time relation with reference to each other such that predetermined amplitude variations of the several signal voltages occur substantially in timephaserelation. More speciiically, the third amplitude variation of signal ecs, the second amplitude variation of signal eze, and the rst amplitude variation of signalferz occur in timephase relation. These signals add up across load impedance 3i, producing an output signal at terminals 22, 22. as represented by curve en, having one amplitude variation or pulse of such magnitude as to exceed the clipping level c of clipping ampliner Iii. This amplitude variation of the output signal corresponds in time with the above-described in-phase components of the individual signals obtained at taps 25, 26 and 2l and produces a control eiect which keys multivibrator I5. The multivibrator generates and applies to input terminals di, di, of unit Ml a single unidirectional pulse of substantially recamarti tangular wave form. The time-delay characteristics oi transmission line llt, cause taps it, at and lil to derive a plurality of signals individually having a wave form corresponding to that of the applied pulse and a time relation with reference to each other such that the signals derived at the individual tapping points appear in a predetermined time sequence. These signals develop across the common impedance di a control signal, represented by curve ce oi Fig.

3, for application to amplifier it. This control signal likewise has pulse-type amplitude variations of equal pulse durations t4 determined by the period of multivibrator la and unequal pulse spacings t5 and te determined by the relative positions of taps d5. d6 and di. The reply signal transmitted from antenna system iii, It in response to this control signal is a pulse-modulated signal comprising components having pulse durations and pulse spacings corresponding to those of control signal etz. In other words, the transponder system including units 'le and Ydu with taps arranged as indicated in the drawvidually having characteristic variations determined by predetermined ones of the characteristic variations of the interrogating signal and a time relation with reference to each other such that predetermined variations ofthe individual characteristics of the plurality of signals occur substantially in time-phase relation. Clipping amplier It and the succeeding units in the system constitute amplitude-selective means responsive jointly to the aforesaid predetermined variations of the plurality mitting a. coded reply signal.

With reference to the particular coded replyof signals for transvsignal transmitted, unit di! comprises means for deriving from the interrogating signal 'a plurality of signals individually having predetermined characteristic variations and a time relation with reference to each other such that predetermined variations of the individual characteristics of the plurality of signals appear in a predetermined time sequence-which may be the same or diierent from the time sequence of the characteristic variations of the-interrogating signal. Furthermore, amplifier IG and wave-signal generator and modulator Il comprise means responsive to the predetermined variations of this plurality of signals for transmitting a coded replyv signal having components corresponding in number to this plurality of signals and a spacing determined by the time seauenceof such plurality of signals.

The coding provisions of the receiver and the transmitter of the described transponder system are helpful in establishing the desired secrecy characteristics and for many installations are adequate. However, it will be noted that while the selective receiver is arranged to respond primarily to interrogating signals having characteristic variations appearing in a predetermined accessi time sequence. an interrogating signal having an exceedingly long pulse duration such as to include substantial portion of the period of the time sequence may, in some instances, cause the transponder to reply. This operation may be avoided by the arrangement of Fig. 4 which represents a modication of unit 2li, the arrangement being designated generally as 2li' and including the present invention.

Unit 20' includes a conventional differentiating circuit 53, and may be of the type illustrated at page 162, Fig. 94 or Principles of Television Engineering, supra. VIt is connected between the input terminals 32, 32, of unit 2li and the input terminals M, 54, of a transversal time-delay lter network comprising a pair of spaced, parallel transmission lines 55, 55, terminated in 'their surge impedance 56. Transmission line b, 55, may have the same general construction and electrical properties as the corresponding elements described above in connection with unit 20. A plurality of pairs of adjustable taps are associated with transmission line 55, E5, three such pairs 51, 51', E8, 58' and 59, 59' being represented in the drawings. Taps 51, -68 and 59 are individually included in circuit with isolating resistors W, 6i and 82, respectively, and a common impedance 63. Taps El', 5B' and W are similarly included in circuit with isolating resistors Bil', tl' and t2' and a common impedance ad'. A phase inverter M, such as a triode amplifier 'with unity gain, is coupled across impedance tu and the signal derived from its output terminals dii, te is combined with the signal developed across impedance 63 and applied to output terminals E2, 52 of unit 20'. The impedance relations of/the isolating resistors and common load impedance of this unit are substantially as described above in the discussion of unit 2li.

In considering the operation of the coding arrangement of Fig. 4, reference is made to the series of curves of Fig. wherein the subscriptsindicate the elements of unit 2b from which the represented signal voltages are derived. It will be assumed that all of the taps illustrated in Fig. 4 are effective, and that the interrogating signal applied to input terminals 3E, ai, from the amplitude limiter i3 has a wave form as represented by curve cs2. This signal has a maximum amplitude limited to predetermined value e, pulsetype amplitude variations of unequal pulse dura- Sill tions designated tuts and t9 and occurring in a predetermined time sequence represented by unequal pulse spacings tio and tu. This interrogating signal, after being differentiated in unit 53, is applied to input terminals 5t, 5d, of the transversal time-delay iilter network with a wave form as represented by curve es'r. A first plurality of signal voltages are derived at taps 5l, 5d and 5b of the delay network, as shown, respectively, by curves est, esa and ess. This plurality of signals individually has a predetermined maximum amplitude equal to the value e if line is assumed to4 have zero attenuation and amplitude variations or impulses determined by both the leading and trailing 'edges of the pulse-type amplitude variations' of the .received interrogating signal. The time-delay characteristics of the network and the selected relative positions of taps 51, 5d and 59, however, cause the signals to have a time .relation with reference toA each other such that relative positions of taps 51, 58 and 59 are so chosen that the signal impulses derived from the trailing edges of the first, second, and third am-a plitude variations of the applied interrogating signal as derived at taps E9, EB and 51, respectively, occur in time-phase or coincident relation.. This plurality of signals develops a signal voltage across impedance 63, which is translated to phase inverter ed appearing at output terminals W thereof as represented by curve een. l

A second plurality of signals is derived at taps di', lid and 59 of the delay network. The lndividual signals of this second plurality of signais likewise have a predetermined maximum amplitude equal to value e and contain amplitude variations or impulses determined by the leading and trailing edgesof the pulse variations of the interrogating signal. Their time relations, due to the effect of the delay filter and the relative positions of taps 5l', 5B' and 58', is such that the signal impulses vderived from the leading edges of the iirst, second and third amplitude variations of the inteirogating signal as obtained,

respectively, at taps 59', 5B' and 51' appear substantially in time-phase or coincident relation. The signal of curve eos' is produced by this second plurality of signals' across commo'n lmfpedance 63'. The series of taps 51', 58' and 59' are further adjusted with reference to the series of taps l, till and 58 so that the ln-phase or coincident impulses of the second plurality of signals occur in 'the same time-phase relation as the irl-phase o1' coincident impulses of the irst plurality of signals. This phase relationship will be clear from. a comparison of curves co3' and ecs. .as a consequence, the Voutput signal obtained at output terminals 52, 62 of the networls, resulting from .the combination of the signais represented by curves ees' and ecs, has the wave form of curve cs2. ping level of clipping amplifier It to a value indicated by a broken-line curve c', a, single control eiect is derived from this output signal for keying multivibrator i5. Thus, the improved coding arrangement of Fig. (l. is critical not only to the time sequence of the amplitude variations of the received interrogating signal, but also to the pulse durations of such amplitude variations.

A. modification of the coding arrangement for the transmitter portion of the transponder system is represented in Fig. 6 and indicated genorally as til. This arrangement is similar in construction and design to unit d0, described above, and corresponding components thefreof are designated by like reference numerals primed. However, unit d0' includes a, second series of adjustable taps de, lili" and M" as well as isolat,

ing resistors lill", fill", and 50" and a common impedance 5i". verter 'l0 is coupled across impedance 5i" and the signal derived in the output circuit thereof, is applied to the input terminals of an integrat.

ing circuit il along with the signal voltage developed across' common impedance 5I". The integrating circuit may be of the type shown at page i63, Fig. of Principles-of Television Engineering, supra. The output terminals 42', d2', of unit fill' are coupled to the output terminals of integrating circuit li.-

To utilize the described modification of the coding arrangement in the transponder system of Fig. i, multivibrator i5 is adjusted to generate o. sharp pulse of rectangular wave form having a pulse duration tu., curve ,f of Fig. 7. This curve demonstrates the time relation of the pulses derived at the individual taps of the transversal By adjusting the clip-l The input circuit of a phase inf plurality of pairs of signals appearing in a, predetermined time sequence and individually comprising unidirectional signals of opposite polarities having a predetermined time separation. That is, for the assumed relative positions of the adjustable taps, the derived pairs of. signals `have individual time separations indicated tio, tr: and tra and appear in a time sequence represented by intervals tis and tze. The integrated signal obtained at output terminals 42', 42', has a wave form as indicated by curve h and, after translation through amplifier I6, controls wave-signal generator and modulator l1 vto transmit a, reply signal having a modulation envelope of similar wave form. Thus, unit 40' permits the coded reply signal to have components of predetermined pulse duration and pulse spacings as determined by the selective positioning of the adjustable taps associated with transmission line llt', 43'.

The described coding arrangements are extremely iiexible and provide a degree of secrecy which increases with the complexities of the particular arrangement utilized. By increasing the number of taps utilized in unit 20, for example, the receiver may be selectively responsive to an interrogating signal containing spaced groups of many more than three amplitude variations appearing within each group in a predetermined time sequence. The receiver arrangement :may be further modified to provide a higher degree of secrecy by utilizing an increased number of pairs of taps in the manner described in connection with the arrangement of Fig. 4 so that the receiverV is responsive to the duration as well as the spacings, or time sequence, of the amplitude variations ofthe interrogating signal. In like manner, the secrecy features of the transmitter portion may be varied by including any desired number of tapsor pairs of taps in the coding arrangement of units Ml and dll. respectively, spaced in accordance with any desired coding schedule.

Although it is preferred that coding arrangements be associated with both the receiver and transmitter portions of the transponder system, veither one may be omitted. Where the coding feature is utilized in the transmitter but omitted from the receiver, a complete coded answer is transmitted in response to a single pulse modulation of the interrogating signal.

In any case, where coding of the transmitted reply signal is employed, it is desirable to provide a holding or blocking feature in the receiver to disable the receiver for such a period of time' i ti The described transversal timendelay `iilter networks are especially useful in the coding arrangements under consideration since they permit the Coding schedules to be altered throughv the simple expedient of adjusting the number or relative positions of the adjustable taps. The maximum time delay desired to be obtained in the network governs the lengths of the transmission lines of the described networks.

In the foregoing explanation oi the invention, the interrogating signal has been assumed to have amplitude variations appearing in a predetermined sequence. The invention is equally applicable to transponder systems designed to receive interrogating signals having pulse-type frequency or phase variations appearing in a predel termined time sequence. fllciereiore, the expression an interrogating signal having predetermined characteristic varlations appearing in a predetermined time sequence, as used in the description and the appended claims, is intended to include all such applications.

It will be apparent from the described operation of units .2u and 20 and particularly from the curves of Figs. 2 and 5, that the amplitude-limiting feature of a transponder system including a coded receiver in accordance with the invention results in an improved signal-to-noise raitio. This feature results from the fact that noise signals being of a""random nature will, in general, fall below the clipping level of ampliiier ifi, and thus be ineiective to cause a response from the system.

It will .be understood that multivibrator it is a desired, but not absolutely necessary component of the transponder system. By including this component in the system, the pulse-type characteristic variations of the transmitted reply signal. may have any desired pulse durations, even though the coding arrangement ci Fig. ii he omitted. Otherwise, installations which lack. titansn1 mitter coding facilities similar to that of Fig. 6 transmit answer signals having .characteristic variations of puise durations substantially the same as that of the interrogating signal.

While there have been described what are at present considered to loe the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and medincations may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. In a wave-signal transponder system for utilizing an interrogating signal having a pluralty of pulse-type characteristic variations of predeter-` mined pulse durations occurring in a predetermined time sequence, a selec'idve receivlng system comprising, time-delay means, means associated with said time-delay means for deriving from said interrogating signal a first corresponding plural- .ity of signal impulses limited to predetermined amar/ii ing edge of a different predetermined one of said pulse-type characteristic varia/tions oi.' said interrogating signal, and amplitude-selective means unresponsive to signal impulses of said predetermined maximum amplitudes but responsive jointly to said coincident ilrst and second plurality of signal impulses for deriving an output signal from said receiving system, whereby said receiving syst'em is responsive only to interrogatlng `signals having a plurality of pulse-type characteristic variations of said predetermined pulse durations occurring in said predetermined time sequence.

2.` A .selective wave-signal translating system for translating a signal having a plurality of pulse-type amplitude varia/tions of predetermined pulse durations occurring in a predetermined time sequence comprising, an input circuit for receiving said signal, means coupled to said input circuit for deriving from said received signal a rst corresponding pluralityo'f signal impulses occurring in time coincidence and individually represen-tingr the leading edge of adierent one of said pulsetype amplitude variations of said received signal, means for deriving from said received signal a second corresponding plurality of signal impulses occurring in time coincldencewith one another and with said first plurality of signal impulses and individually representing the trailing edge of a different one of said pulse-type amplitude variations of said received signal, and means responsive jointly to said coincident first and second plurality of signal impulses for deriving an output signal, whereby said translating system is responsive only to signals having pulse-type amplitude variations of said predetermined pulse durations occurring in said predetermined time sequence.

3. A selective wave-signal translating system for translating a signal having a plurality of pulse-type. amplitude variations of predetermined pulse durations occurring in a predetermined time sequence comprising, an input circuit for receiving said signal, means coupled to said input circuit for deriving from said received signal a rst corresponding plurality of .signal impulses limited to a predetermined maximum amplitude, occurring in time coincidence and individually representing the leading edge of'a different one of said pulse-type amplitude variations of said received signal, means for deriving from said received signal a second corresponding plurality of signal impulses limited to a predetermined maximum amplitude, occurring in time coincidence with one another and with said first plurality of signal impulses and individually representing the trailing edge of a different one of said pulse-type amplitude variations of said received signal, and means unresponsive to signal impulses of said predetermined maximum amplitudes but responsive jointly to said coincident first and second plurality of signal impulses for deriving an output signal, Whereby said translating system is responsive only to signals having pulse-type amplitude variations of said predetermined pulse durations occurring in said predetermined time sequence.

, 4. A selective wave-signal translating system fpr translating a signal having a plurality of pulse-type amplitude variations of predetermined pulse durations occurring in a predetermined time sequence comprising, an input circuit for receiving said signal,` means for applying said signal to said input circuit with a maximum amplitude limited to a predetermined value, means coupled to said input circuit for' deriving from said received signal a first corresponding pluralitily oi? signal impulses limited to said predetermined maximum amplitude, occurring in time coincidence and individually representing the leading edge ci a diiierent one of said pulse-type amplitude variations of said received signal, means for deriving from said received signal a second corresponding plurality' of signal impulses ilelted to said predetermined maximum amplitude, 'occurring in time coincidence with one another and with said first plurality of signal imand individually representing the trailing ci a different one of said pulse-type amplivariations of said received signal, and means unresponsive to signal impulses of said predetermined maximum amplitude but responsive jointly 'd coincident first and second plurality of for deriving an output signal, wlicreiiy said translating system is responsive only to signals having pulse-type amplitude variations ci? said predetermined pulse durations occurring in said predetermined time sequence.

A selective wave-signal translating system for translating a signal having a plurality of pulse-type amplitude variations of predetermined pulse durations occurring in a predetermined time sequence comprising, means including a t ,-delay network for receiving said signal, ra .ns coupled to time-delay network for from. said received signal a rst corresp iding plurality of signal impulses occurring :une coincidence and individually representing the leading edge of a diierent one of said pulse-type amplitude variations of said received signal, means also coupled to -said time-delay network for deriving from said received signal a second corresponding plurality of signal impulses occurring in time coincidence with one another and with. said first plurality of signal impulses and individually representing the trailing edge dit of different one of said pulse-type amplitude variations of said received signal, and means responsive jointly to said coincident first and second plurality of signal impulses for deriving an output signal, whereby said translating system is responsive only to signals having pulsetype amplitude variations of said predetermined pulse durations occurring in said predetermined time sequence.

i selective wave-signal translating system for translating a signal having a plurality of pulse-type amplitude variations of predetermined pulce durations occurring in a predetermined time sequence comprising, means including a dicrentiating circuit and a serially connected time-delay network; for receiving said signal, means coupled to said time-delay network for deriving from said received signal arst corresponding plurality of signal impulses occurring in. time coincidence and individually representing the leading edge of a different one of said pulse-type amplitude variations of said received signal, means also coupled to said time-delay networl: for deriving from said received signal a second corresponding plurality of signal impulses occurring in time coincidence with one another and with said first plurality of signal impulses and individually representing the trailing edge ol a different one of said pulse-type amplitude variations of said received signal, and means GSllonsive jOintly to said coincident rst and second plurality of signal impulses for deriving 5 an output signal, whereby said translating system is responsive only tosignals having pulsetype amplitude variations of said predetermined 19 pulse durations occurring in said predetermined time sequence.

7. A selective wave-signal translating system for translating a signal having a plurality of pulse-type amplitude variations of predetermined pulse durations occurring in a predetermined time sequence comprising, means including a differentiating circuit and a serially connected time-delay network for receiving said signal, a rst series of taps associated with said timedelay network for deriving from said received signal a first correspondingplurality of signal impulses occurring in time coincidence and individually representing the leading edge of a different one of said pulse-type amplitude variations of said received signal, a second series of taps also associated with said time-delay network for deriving from said received signal a second corresponding plurality of signal impulses occurring in time coincidence Iwith one another and with said first plurality of signal impulses and individually representing the trailing edge of a different one of said pulse-type amplitude variations of said received signal, and means responsive jointly to said coincident rst and second plurality of signal impulses for deriving an output signal, whereby said translating system is responsive only to signals having pulse-type amplitude variations of said predetermined pulse M durations occurring in said predetermined time Sequence.

BERNARD D. LOUGHLIN.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS l0 Number Name Date 1,142,252 Harkness June 8, 1915 1,172,526 Harlow Feb. 22, 1916 1,647,284 Field Nov. 1, 1927 1,812,187 Ackerly June 30, 1931 15 2,172,354 Blumlein Sept. 12, 1939 2,211,942 White Aug. 20, 1940 2,229,097 Koenig Jan. 21, 1941 2,229,249 Lewis Jan. 21, 1941 2,266,401 Reeves Dec. 16, 1941 20 2,275,930 Torcheux Mar. 10, 1942 2,311,445 Kerr Feb. 16, 1943 2,403,561 Smith July 9, 1946 2,418,127 Labin Apr. 1, 1947 s FOREIGN PATENTS Number Country Date 463,694 Great Britain Apr. 2, 1937 528,192 Great Britain Oct. 24, 1940 

